The unusual thing about Steam is that their high fees discourage developers from using it. If they charged 3% then everyone would use it, there would be a million games there and being in the store wouldn't cause you to stand out at all.
If they charge 30%, most small developers don't use it and then if you choose to pay, you get to be featured on a list without that many of your competitors. You're paying for exclusivity. Then you reach an equilibrium where small developers pay to be listed next to Valve's first party AAA titles, until there are enough of them that the exclusivity is sufficiently diluted to stop attracting more developers.
That doesn't apply to Apple because anyone who wants to reach iOS customers doesn't have any reasonable alternative way to do it, so there are millions rather than thousands of apps in the store and the exclusivity of being listed is already diluted to nothing. But they still charge the same rate.
I do think Steam can charge more because it offers more good services for users than any other app/game store: search/discovery features that have gotten better over time, per game communities/mods, tons of social features including a marketplace, multiple platform support, and their customer service. Sure their fee might discourage some developers, but if there is a case to be made for 30%, this is it. It all works because there is competition between Steam/GOG/Itch/Epic/etc.. on multiple open platforms in a pretty well differentiated way.
If they charge 30%, most small developers don't use it and then if you choose to pay, you get to be featured on a list without that many of your competitors. You're paying for exclusivity. Then you reach an equilibrium where small developers pay to be listed next to Valve's first party AAA titles, until there are enough of them that the exclusivity is sufficiently diluted to stop attracting more developers.
That doesn't apply to Apple because anyone who wants to reach iOS customers doesn't have any reasonable alternative way to do it, so there are millions rather than thousands of apps in the store and the exclusivity of being listed is already diluted to nothing. But they still charge the same rate.